Concrete footing size on a cylindrical silo

[drujon]

I have a 30ft x 6'-5dia Silo that weighs 10,000lbs empty (dead load) and 16,500lbs (live Load) when in use. The wind speed in the area is 120 mph. I need to get a rough estimate the concrete slab size for bidding purposes only.

I looked through different engineering handbooks and on this site, but I cant find a good formula to use for this.

i used this formula for wind force but i'm not sure what to do with it.

Force = A × P × Cd
A = projected area of the item (cylinder = 2?rl)
P = wind pressure (lb/ft2) = .00256 x V^2 (V= wind speed in mi/hr)
Cd = Drag coefficient = 2.0 for flat plates. For a long cylinder (like most antenna tubes), Cd = 1.2. Note the relationship between them is 1.2/2 = 0.6, not quite 2/3

F= (2? x30’x3.25’)ft^2 x (.00256x120^2 mph)lb/ft^2 x (1.2) = 27,099.9 lbs

 

[drujon]

that really 6'-6" dia

 

[HeavyCivil]

Are you sure the live load isn't higher?

 

[drujon]

no that's it's actually a scrubber/ cooler. It will just have some water running through it. I just called it a silo to be generic.

 

[csd72]

You can sometimes get into real trouble bidding on these things without a geotechnical report as these things put a high bearing pressure on the soil and could sometimes require piles.

Also, are you in a seismic region?

I suggest you talk to a local structural engineer to get a preliminary design based on local conditions.

 

[drujon]

it will be located in north Carolina. I do not have soil or seismic information at this time. i'm just trying to get a general idea of size or amount of concrete.

 

[BAretired]

Why does A include 2*pi?

BA

 

[drujon]

projected area of a cylinder = 2?*r*l

 

[BAretired]

I would have thought that projected area, A = d*l where d is diameter and l is length. Why is pi included?

BA

 

[drujon]

i just did a search for projected area of a cylinder. that what i came up with

 

[drujon]

i thought this would be an easy question for someone who has done this before. all I'm looking for is some rule of thumb so i can get a round about number at this point. I just need to get in the right range

 

[hokie66]

drujon,
In wind terms, the projected area is just the width x height. What you used is the surface area all around the cylinder.

You should not be using this site for preparing bids based on guesswork. That may get you into lots of trouble.

 

[BAretired]

2*pi*r*l is the area if a rectangular piece of material needed to make a cylinder of length l and radius r. By including it in your formula, you are off by a factor of pi. Otherwise, your approach seems correct.

So A = 6.5 * 30 = 195 SF

p = 0.00256 * 120^2 = 36.9 psf

F = 195 * 36.9 * 1.2 = 8630# acting 15' above the base.

M = 8630 * 15 = 130,000'#

BA

 

[SAIL3]

I would usually use a Cd=0.8, but 1.2 is ok for bid purposes.
As csd72 pointed out that if one has questionable soil requiring piles then the cost could be much much higher than the following
normal setup.
Assune soil capacity to be 2000psf.
Size the slab to resist the overturning moment with a safety factor of 1.5....this gives a min. value.
Then check soil pressure at toe, if it is less than 2000psf then you are ok with min. size.
If it is greater then the soil allowable, then increase size of slab untill you get below the allowable soil press.
It all hinges on the assumption of the allowable soil press.
Perhaps you can include with your bid a statement to that effect, absence any soil info.

 

[cvg]

you can get soil information here and this would be better than just assuming something:

http://websoilsurvey.nrcs.usda.gov/app/